This invention relates to a solid state area imaging apparatus which converts a two dimensional optical image into an electrical signal.
An ordinary area imaging apparatus employing a photodiode array is shown, for instance, in the WESCON Tech. papers, session 13-2 August 1967. Such an apparatus consists of photodiodes, switching elements such as MOS transistors and scanning shift registers. In such an apparatus, an electric charge, which is proportional to the intensity of the light incident upon each diode, is stored in each diode and is read out when the switching element which is connected to the diode is turned on by being addressed by both a vertical scanning shift register and a horizontal scanning shift register. A sequential video signal representing a black and white image of an object is obtained as an output of this apparatus.
The advantages of a X-Y addressed area imaging apparatus, such as that described above, are less blooming, a high sensitivity, a high quantum efficiency for light of short wave lengths (450 nm) and less dark current, such advantages being obtained by utilizing a p-n junction as a photosensor.
On the other hand, the disadvantages of a X-Y addressed area imaging apparatus include the existance of fixed pattern noise (FPN) caused by the non-uniformity of the horizontal multiplex switches, and a high random noise level which is caused by the large output capacitor and which increases in proportion to increases in the number of picture elements.
With respect to an area imaging apparatus which operates by means of charge transfer techniques, two methods, namely, the frame transfer method and the interline transfer method, have been proposed. An area imaging apparatus employing the frame transfer method is shown, for instance, in IEEE Trans. on Electron Device, ED-20, No. 3, p244, 1973. Such a frame transfer imaging apparatus, for example, consists of a photosensitive CCD array, a temporary storage CCD array and an output CCD register. A pattern of stored charges in the CCD array which represents an input optical image and which is formed by the entirety of the charges stored during one field interval is transferred into the temporary storage array during a vertical blanking period. Then, the pattern is shifted one horizontal line at a time into the output CCD register during a horizontal blanking period, and is read out by a high speed horizontal clock to produce a sequential video signal.
An imaging apparatus employing the interline transfer method, which is shown, for instance, in 1974 IEEE Intercon Tech. papers, session 2-2, consists of alternate columns of photosensitive elements, vertical transfer CCD registers and an output CCD register. During a vertical blanking period, signal charges detected which are by the photosensitive elements are transferred into the vertical transfer registers. These signal charges are then shifted one horizontal line at a time into the output CCD register during a horizontal blanking period, and transferred out by a horizontal clock.
Advantages of such charge transfer area imaging apparatus are a low FPN arising from the dark current which increases at a high temperature and less random noise due to less output capacitance.
On the other hand, disadvantages thereof are decreases in the output signal, which is caused by transfer losses, a small maximum transferable charge value and poor blooming and smearing characteristics.